1. Technical Field
The invention concerns an apparatus for reducing the transmission capacity required to transmit signals in bearer channels and in signalling channels of an integrated services digital network.
2. Description of the Related Art
When signals are transmitted via an integrated services digital network, a so-called ISDN, the transmission capacities of the individual channels are predetermined and cannot be changed. A bearer channel for transmitting voice and data signals, the so-called B-channel has a transmission capacity of 64 kbit/s. A channel for transmitting signalling signals, the so-called D-channel has a transmission capacity of 16 kbit/s. Standardized transmission in the basic access configuration provides for two B-channels and one D-channel. Two independent integrated services digital networks can be interconnected via a radio link for example. Only a limited number of channels are available in the radio link. The transmission capacity of the individual channels is not predetermined.
Alcatel's 1994 product brochure "Alcatel 9800" describes a subscriber access system whereby a digital telephone network can be connected to another digital telephone network and/or to a radio network. 1024 terminal stations for example can be connected to the digital telephone network via 30 transmission channels of 64 kbit/s each, or via 60 transmission channels of 32 kbit/s each. The distribution into 30 or 60 transmission channels is predetermined. The system which contains 60 transmission channels operates in accordance with the method of adaptive differential pulse code modulation, which can be found for example in ITU-T recommendation G. 726, and only supports voice services. An ISDN link for a basic access configuration requires two 64 kbit/s channels and one 16 kbit/s channel to support all ISDN service features. These channels must be provided via radio.